This invention relates to a novel process in which a protective coating is generated on specific areas of porous metallic components of orthopaedic implantable prostheses, such as tibial tray components, hip stems, and acetabular cup shells of various sizes. Many orthopaedic implants have porous surfaces on at least a portion of the prosthesis, with high surface areas, in order to provide a site for bone to grow into the prosthesis which results in a high degree of device stability and fixation to the particular joint site. The manufacture of such porous components is quite complicated due to the need to protect such porous regions from damage or contamination during the multi-step manufacturing processes.
Typical of these manufacturing steps is machining which produces fine metallic particulates, dust, etc. which could be embedded in the valleys of the porous surface. Another step which may be used in the manufacture of the prosthesis is sandblasting. Again, a consequence of this step may include the production of fine debris which could be trapped within the pores of the metallic surface.
For these reasons, the need for a protective coating is essential for the successful production of high quality orthopaedic implants. A suitable coating material has two conflicting requirements. On the one hand, the coating must withstand the harsh effects of the coolant used in the machining step which lasts for at least one hour, as well as the severe force of the sandblasting step, lasting 10 minutes at pressures ranging from 60 to 100 psi. On the other hand, the ideal coating should also be amenable to easy removal using mild and environmentally safe reagents.
Previously, polymethyl methacrylate (PMMA) coatings were applied to the porous surfaces of implantable prosthesis from a 50/50 mixture of acetone/toluene solutions. After application to the porous surface, the solvent mixture was allowed to evaporate, leaving a PMMA layer behind. This process required the use of a fume hood, careful avoidance of spilling of solvents, and avoidance of exposure of these flammable solvents to open flames, sparks or other hazards. PMMA solutions had to be prepared just prior to the coating step, due to the high volatility of the solvents, which could result in changes in PMMA concentration. Another extra cumbersome requirement with the currently used PMMA system is the need to prepare different PMMA solution concentrations for different implants because of the difference in the porous structure of the implant. For example, for hip products a ratio of 30 grams PMMA to 85 ml of solvent is used, whereas in the case of tibial trays, a ratio of 24 grams PMMA to 85 ml solvent is sufficient. Furthermore, for femoral knee components 30 grams PMMA to 100 ml of solvent is used. Each of these preparations require at least 24 hours to ensure complete dissolution of the PMMA in the solvent mixture. These PMMA solutions are usually applied via pneumatic driven syringes where clogging due to premature evaporation of the solvents often occurs. The preparation of the PMMA solution must be conducted in a well ventilated hood and the application of the solution to the prosthetic part is conducted in the hood.
Clearly, the use of a PMMA solution system presents difficulties and a more efficient, environmentally safe, and cost effective approach is desirable.